The GSM standard currently used in mobile communication involves the use of “GSMK” (Gaussian Minimum Shift Keying) Modulation, which uses a signal space having signal points which have a phase difference of 180°. To this end, the packet service GPRS (General Packet Radio Service) has been developed, which allows operation at higher data rates. Another standard currently used is the TIA/EIA-136 (IS 136) standard, which involves the use of a π/4-DQPSK (D-quaternary-PSK) modulation method for generating the transmission signals. To a certain extent as a transitional standard between GSM and GPRS, on the one hand, and UMTS, on the other, the EDGE standard and also the associated EGPRS (enhanced GPRS) packet service have been defined. Although EDGE is a TDMA (Time Division Multiple Access) method, there is already a transition from GMSK modulation to 8-PSK modulation. 8-PSK modulation involves the use of a signal space having 8 signal points, the phase difference between the individual signal points being 45°. By contrast, UMTS uses the CDMA (Code Division Multiple Access) method (also in combination with TDMA and/or FDMA), which involves the signals in any radio link being encrypted using a respective proprietary code, the “spread code”.
One general aim is to develop mobile communication appliances which are designed for operation with a plurality of different mobile radio standards and which may accordingly be used in the various mobile radio systems. One problem in this context, however, is the fact that the modulation methods described above are based on different symbol clock frequencies which cannot be converted into one another by integer factors or simple rational ratios. Consequently, at least two different system clocks are normally required which are usually derived from separate crystal oscillators.
Previous transmission apparatuses for combined UMTS and GSM/EDGE applications thus have exclusively a plurality of crystals or PLL (Phase Locked Loop) circuits which supply system clock frequencies precisely co-ordinated with the respective standard which are usually an integer multiple of the inverse symbol period in the transmission method. The architecture of the signal processing lines is rigidly linked to these frequencies. Both the physical operating clock and the physical sampling clock are thus not independent of the standard-specific digital signal processing, particularly the standard-specific symbol period, in these transmission apparatuses.
German patent application DE 100 45 547.6 “Verfahren zur systemunabhangigen digitalen Erzeugung von Mobilkommunikations-Sendesignalen verschiedener Mobilfunkstandards” [Method for system-independent digital generation of mobile communication transmission signals in various mobile radio standards], which is prior art in line with §3(2) of the German Patent Act, describes a related method which, by way of example, is not designed for the specific requirements of the UMTS standard, however, and, in particular, has the property that it not only uses a single crystal oscillator but also makes joint use of the same digital/analog converter (DAC) and other circuit components for a plurality of standards, and also operates the DAC at the same sampling frequency for all of the standards, which is not advantageous for a system which supports the UMTS and GSM/EDGE standards, for example.